As is known in the art, analog-to-digital converters (ADCs) convert a signal in analog format to a signal in digital format. Conventional ADC circuits can have a variety of circuit architectures each of which has certain concomitant disadvantages. Known ADC architectures include pipeline, sigma-delta, cyclic, flash, successive approximation, and dual-slope. Each ADC architecture is generally applicable to a limited operating range. That is, each of these architectures has strengths and weaknesses that make them more amenable to working in certain frequency and resolution ranges.
Some ADC architectures do not operate outside certain ranges or consume prohibitively high power in certain ranges as compared to other architectures. Even within preferred operating ranges, a given architecture can have a performance level that is dictated by certain circuit parameters that are fixed for a given design.
High-performance analog-to-digital converters (ADCs) are generally optimized for conversion speed and resolution with a given size and power budget. In CMOS-based mobile biochemical sensor application, however, speed and resolution are immaterial because of the slow reaction rates (>seconds) and inherent experimental errors (˜10%) typical of most biochemical reactions. Instead, ADC sensitivity, power consumption and size may be of greater interest.
In one known attempt to apply ADCs to biochemical reactions, to convert sub-nA level photo currents into voltage, the input signal is amplified using large-gain (106) current mirrors, increasing power and area requirements and the current mirror's susceptibility to mismatch errors for a given chip area, as disclosed in U. Lu, Hu, B. C-P., Shih, Y-C., Wu, C-Y, and Yang, Y-S, “The design of a novel complementary metal oxide semiconductor detection system for biochemical luminescence,” Biosensors and Bioelectronics, vol. 19, pp. 1185-1191, 2004, which is incorporated herein by reference.
In M. Simpson, Sayler, G, Patterson, G, Nivens, E, Bolton, E., Rochells, J., Arnott, J, Applegate, B., Ripp, S., and Guillom, M., “An integrated CMOS microluminometer for low-level luminescence sensing in the bioluminescent bioreporter integrated circuit,” Sensors and Actuators B, vol. 72, pp. 134-140, 2002, which is incorporated herein by reference, increased ADC sensitivity was achieved by successive capacitive integration and voltage-to-frequency conversion at the expense of increased power consumption and long conversion time (˜ seconds).